The present invention relates to a device and a process for the production of a reaction mixture which forms plastic material (and, in particular, foamed plastic material) from two flowable reaction components, comprising storage containers and supply lines which lead via at least one servo-controlled inlet member (which generally consists of a control piston, an inlet valve, and/or a nozzle) into a mixing chamber, wherein the inlet member is assigned a sliding valve, the slide of which is assigned a moving electromagnet and a restoring spring, as known from U.S. Pat. No. 3,788,337.
Devices of this type are used in particular in polyurethane processing technology, wherein the required mixture quantity is produced in continuous fashion. The time required to form the mixture quantity is referred to as "shot time". For small mixture quantities the shot time is correspondingly short since the dispensing pumps operate with approximate delivered quantities of 50 to 100 liters/minute.
The inlet members are generally hydraulically operated, for which purpose sliding valves, which can be switched electromagnetically, are actuated. The end of the slide has the form of a magnetic core and is guided in an exciting winding, or an iron core cooperating with the slide is fixed in the exciting winding. The build-up of the magnetic field requires a certain length of time and the magnetic force must first overcome the opposing prestressing force of the restoring spring before the sliding valve commences the opening procedure. It is only then that the actual opening procedure commences, which, due to the continued build-up of the magnetic field in association with the opposing force corresponding to the spring characteristic, requires a relatively long period of time and proceeds in an unstable fashion. With a shot time of, for example, 0.4 sec, the shot time tolerance, which is governed by the build-up of the magnetic field, amounts to approximately 0.02 sec, resulting in a fluctuation in the component quantities of .+-.5%. If, for example, an object is produced by in situ foaming from polyurethane foamed material, too large a mixture quantity leads to a higher density than desired. Too small a mixture quantity leads to too low a density, or in extreme circumstances the mold cavity is not entirely filled. If, in the case of separately controlled inlet members for the reaction components, the opening procedures of these inlet members differ from one another, the mixture ratio is also adulterated during the initial phase of the opening procedure. An adulterated mixture ratio of the reaction components can undesirably change the physical properties of the end product.
The aim of the present invention was to discover a device and a process with which the opening procedure of the inlet members can be monitored more easily and can be reduced in length in order to ensure a precise mixture quantity and a precise mixture ratio.